libstdc++: Call predicate with non-const values in std::erase_if [PR107850]
As specified in the standard, the predicate for std::erase_if has to be
invocable as non-const with a non-const lvalues argument. Restore
support for predicates that only accept non-const arguments.
It's not strictly nevessary to change it for the set and unordered_set
overloads, because they only give const access to the elements anyway.
I've done it for them too just to keep them all consistent.